1. Field of the Invention
The present invention relates to a gas sensor configured to detect a gas. The present invention relates to a method for manufacturing the gas sensor configured to detect a gas.
2. Description of the Related Art
A gas sensor configured to detect a gas in an atmosphere is used in various fields from household to industrial products, such as an air contamination monitor including a gas-leakage alarm or an air purifier, a function of automatic ventilation control of an automobile, and a breath analyzer.
A generally-known gas sensor is a resistant gas sensor using a resistance change due to an adsorption reaction or the like of a gas included in a metal oxide conductor.
A humidity sensor, which is one of gas sensors can detect moisture (water vapor) in an atmosphere and is used for a wide range of application from an air conditioning apparatus used for cooling and heating, humidification, dehumidification, and the like, a refrigerator, and a clothes dryer, to a weather reconnaissance aircraft, and a medical device.
Known humidity sensors are a resistant humidity sensor using a resistance change of metal or metal oxide conductor due to a difference in humidity (Patent Document 1), a capacitive humidity sensor using a permittivity change of an insulating film provided between electrodes (Patent Document 2), and the like.
A gas sensor generally has a structure in which a detection portion where electrical characteristics (e.g., electric resistance or permittivity) are changed by the gas adsorption action or the like and a circuit portion that converts the change of the electrical characteristics into an electrical signal (e.g., voltage change) and outputs the electrical signal are combined.
On the other hand, a technique in which a semiconductor device is formed using a semiconductor film formed over a substrate is known. For example, a technique in which a transistor is formed over a glass substrate using a thin film containing a silicon-based semiconductor material is known.
Amorphous silicon, polycrystalline silicon, and the like are known as semiconductor materials. Although transistors including amorphous silicon have low field effect mobility, a larger substrate can be used in the case of using amorphous silicon. Meanwhile, although transistors including polycrystalline silicon have high field-effect mobility, they need to be subjected to a crystallization step such as laser annealing and have characteristics such that they are not always suitable for larger substrates.
As another material, an oxide semiconductor has attracted attention recently. For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Document 3).